1. Field of the Invention
The present invention relates to an isolated switching power supply apparatus, and particularly to an isolated switching power supply apparatus in which a controlled variable can be transferred from the secondary side to the primary side and direct control can be performed without using a photocoupler, and further which has good transient reactivity.
2. Description of the Related Art
FIG. 10 illustrates a circuit diagram (Conventional Example 1) of a conventional isolated switching power supply apparatus. The Conventional Example 1 is a single transistor type resonance reset forward converter having a fixed voltage output, using a conventional feedback method for the output voltage.
With the isolated switching power supply apparatus 100 shown in FIG. 10, a power transformer 3 includes a primary winding 3A and a secondary winding 3B. One end of the primary winding 3A is connected to a positive side input terminal 1, and the other side is connected to a negative side input terminal 2 via a power switch 4. The gate of the power switch 4 is connected to a PWM control IC 69.
The PWM control IC 69, which is a circuit for turning the power switch on and off, is provided at the primary side, and includes a comparator 71 and a ramp voltage waveform generating circuit 70 connected to the inverting input terminal thereof. A serial circuit including a resistor 73 and a photo-transistor 74B is connected between a DC power source 72 and the negative side input terminal 2, with the contact point therebetween being connected to the non-inverting input terminal thereof.
One end of the secondary winding 3B is connected to a positive side output terminal 10, and the other end is connected to a negative side output terminal 11 via a rectification side synchronous rectifier 5 and choke coil 8 in that order. A smoothing capacitor 9 is connected between the positive side output terminal 10 and the negative side output terminal 11. A commutation side synchronous rectifier 6 is connected between the contact point between the rectification side synchronous rectifier 5 and choke coil 8, and one end of the secondary winding 3B. The gates of the rectification side synchronous rectifier 5 and commutation side synchronous rectifier 6 are connected to a synchronous rectifier driving circuit 7.
Connected between the positive side output terminal 10 and the negative side output terminal 11 are a serial circuit including a light-emitting diode 74A and a shunt regulator 75 and a serial circuit including a resistor 160 and a resistor 162, with the contact point between the resistor 160 and the resistor 162 being connected to the gate of the shunt regulator 75. The secondary side light-emitting diode 74A and the primary side photo-transistor 74B define the photocoupler 74.
With the isolated switching power supply apparatus 100, the power switch 4 connected via the primary winding 3A of the power transformer 3 switches the DC input voltage applied between the positive side input terminal 1 and negative side input terminal 2 so as to be converted into AC. The power transformer 3 transfers power from the primary winding 3A to the secondary winding 3B, rectification is performed at the rectification side synchronous rectifier 5 and commutation side synchronous rectifier 6, and smoothed at an output filter configured of the choke coil 8 and capacitor 9, thereby converting into DC again and outputting DC voltage from the positive side output terminal 10 and the negative side output terminal 11.
Regarding feedback of the output voltage, the voltage divided at the resistors 160 and 162 is compared to a reference voltage of the shunt regulator 75 and an error margin is generated as a DC signal, and the error signal is transferred in the DC state from the secondary side to the primary side by the photocoupler 74. At the primary side, the error signal is input to the PWM control IC 69, compared to the ramp voltage waveform generated by the ramp voltage waveform generating circuit 70 by the comparator 71, and a power switch driving signal which is a square wave subjected to PWM modulation is generated. The power switch 4 is driven following the on/off timing of the power switch driving signals, thereby stabilizing the output voltage, which is the controlled variable, to a specific voltage value.
FIG. 11 illustrates another circuit diagram (Conventional Example 2) of a conventional switching power supply apparatus. The Conventional Example 2 is shown in FIG. 8 in Japanese Unexamined Patent Application Publication No. 2004-208440 (Patent Document 1). The control method with the Conventional Example 2 is called hysteresis control, ripple control, or bang-bang control, and is generally known as a method with excellent responsivity to sudden fluctuation in input voltage and output voltage. Traditional hysteresis control determines the on-duty ratio of the power switch using output voltage ripple as the ramp voltage, and accordingly, the properties thereof tend to be dependent on the ESR (Equivalent Series Resistance) or ESL (Equivalent Series Inductance) of the smoothing capacitor. However, the Conventional Example 2 alleviates the effects with output by superimposing an integrated waveform of the comparator output on the ripple voltage.
While traditional hysteresis control is primarily used with non-isolated switching power sources, the circuit in FIG. 8 of Patent Document 1 applies hysteresis control to isolated switching power source, by driving the primary side power switch from the secondary side via isolation components such as driving transformer, capacitor. The Conventional Example 2 is an example wherein hysteresis control has been applied to a forward converter.
FIG. 12 illustrates yet another circuit diagram (Conventional Example 3) of a conventional switching power supply apparatus. The static regulation properties thereof are shown in FIGS. 13A and 13B. The positive half illustrates the region where current flows from the input side to the output side, and the negative illustrates the reverse current region where current flows from the output side to the input side. The Conventional Example 3 is disclosed in Japanese Unexamined Patent Application Publication No. 2003-88114 (Patent Document 2), with FIG. 12 and FIGS. 13A and 13B being respectively shown in FIG. 8 and FIG. 5 of Patent Document 2.
The Conventional Example 3 is a circuit which suppresses the amount of the current flowing reversely from the output side to the input side of an indirect control isolated switching power source using a synchronous rectifier. In the period wherein the reverse current flows reversely through the parasitic diode of the power switch, a period is created wherein the drain voltage is maintained at approximately 0 V even after the power switch driving signal is turned off, such that detection of a state wherein the gate voltage and drain voltage are both at the L level determines the state to be a reverse current state, and protective operations are performed so that the reverse current does not increase unless the output voltage is further increased, thereby obtaining static regulation properties such as shown in FIGS. 13A and 13B. The reverse current suppression circuit shown in the Conventional Example 3 suppresses the current amount with respect to reverse current wherein the current balance between isolated switching power sources operating in parallel collapses and current flows reversely from the output of one to the output of the other, and to reverse current occurring due to the stored charge in the smoothing capacitor following transient increase in output voltage due to rapid change in input or rapid change in load.
The photocoupler for transferring the error signals from the secondary circuit to the primary circuit in the Conventional Example 1 generally has an absolute maximum rated temperature of around 100° C., and accordingly, cannot be used with switching power sources with a wide temperature range of usage, when taking derating into consideration. The deterioration of CTR (Current Transfer Rate) over time is a problem regarding reliability.
Also, the traditional PWM control used with the Conventional Example 1 has problems of cutoff frequency of the output filter, cutoff frequency of the error amplifier, photocoupler transfer delay, and other problems, and transient responsivity is poor. Accordingly, there is the problem that the output voltage greatly changes with sudden fluctuations in input voltage and output current.
With the Conventional Example 2, an integrated waveform of the comparator output is superimposed on the ripple voltage, but effects of the output filter remain, and the switching frequency fluctuates depending on the output state. For example, adding a low-ESR smoothing capacitor to the output of the switching power source reduces the ripple voltage, such that the switching frequency drops. Also, the switching frequency fluctuates under transient fluctuations in output voltage. This circuit that the switching frequency changes according to the state of use, such that designing an isolated switching power source is difficult.
For example, if the switching frequency of the isolated switching power source fluctuates according to the state of use, there are restrictions in the design of the isolated switching power source, and further, main circuit types which can be used are limited. For example, with a resonance reset forward converter, reduction in frequency generates a surge voltage at the main switch, such that usage thereof is difficult. Also, the input/output filter needs to be designed for the lowest frequency, so a wide range of fluctuation of switching frequency leads to a large-sized input/output filter.
Also, The Configuration Example 2 includes a control circuit at the secondary side and drives the primary side power switch from the secondary side via isolation components, but the startup power for the secondary side control circuit cannot be supplied from the power transformer, such that a secondary startup power supply circuit must be provided, which is a separate path. The secondary startup power supply circuit is substantially a small-capacity isolated switching power source including a switching device, transformer and other elements, and accordingly is a large and expensive component. Further, the driving transformer defining the isolation components for transferring signals for controlling the power switch operate at the switching frequency, such that operation is performed at a relatively low frequency, requiring larger components.
Further, the hysteresis control does not have a circuit component for amplifying error signals, so static regulation properties are inferior to traditional PWM control, and there is the problem that this cannot handle strict stipulations on output voltage precision.
With the Conventional Example 3, a reverse current suppression circuit must be provided separately from the control circuit in order to suppress reverse current, requiring many more components, such that the circuit configuration becomes complex, which is problematic from the perspectives of cost and reduction in size.